E-UTRA (evolved universal terrestrial radio access) according to Release 8 of the 3GPP specifications supports bandwidths up to 20 MHz. However, one of the requirements of future releases of this standard is expected to be the support of bandwidths larger than 20 MHz. A further important requirement on such releases is to assure backward compatibility with Release 8. This should also include spectrum compatibility. That would imply that a future-release carrier, wider than 20 MHz, should appear as a number of Rel-8 carriers to a Rel-8 terminal. Each such carrier can be referred to as a Component Carrier. In particular for early deployments of future releases, it can be expected that there will be a smaller number of future-release terminals compared to many legacy Rel-8 terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband future-release carrier.
The straightforward way to obtain this would be by means of carrier aggregation. Carrier aggregation implies that a future-release terminal can receive multiple component carriers, where the component carriers have, or at least have the possibility of having, the same structure as a Rel-8 carrier. Carrier aggregation is illustrated in FIG. 1 where five component carriers 10, each of 20 MHz bandwidth, have been aggregated together to form an aggregated bandwidth of 100 MHz.
3GPP Release 8, as with many telecommunications standards, makes use of automatic repeat request (ARQ) schemes, and particularly hybrid ARQ (HARQ). Thus, when a receiving terminal correctly decodes a transmission from a transmitting terminal, it responds with a positive acknowledgement (ACK) message. When the receiving terminal incorrectly or unsuccessfully decodes a transmission from the transmitting terminal, it responds with a negative acknowledgement (NACK) message, or alternatively does not respond at all. The transmitting terminal can then retransmit the previously sent transmission. The incorrectly decoded transmission may be discarded; or it may be stored to allow re-combining with the retransmission through techniques known to those skilled in the art. For example, chase combining or incremental redundancy may be employed to increase the probability that the transmission will be successfully decoded when retransmitted and combined with the stored previous transmission.
In Release 8 of the 3GPP specifications, downlink transmissions are dynamically scheduled. That is, in each subframe a radio base station transmits over a control channel control information indicating which terminals are supposed to receive data and upon which resources in the current downlink subframe that data will be transmitted. This control signaling is typically transmitted in the first 1, 2 or 3 symbols in each subframe.
A terminal will thus listen to the control channel, and if it detects a downlink assignment addressed to it, will attempt to decode the data and generate feedback in response to the transmission in the form of an ACK or a NAK (or no response at all) depending on whether the data was decoded correctly or not.
However, no method has so far been specified for transmitting ACK or NAK messages when more than one component carrier are aggregated together in the frequency domain, as shown in FIG. 1, for example.
One possibility for realizing carrier aggregation is to perform coding and hybrid-ARQ retransmissions per component carrier. A straightforward way of realizing this is to transmit multiple acknowledgement messages, one per component carrier. If the number of component carriers in the uplink is at least as large as the number of component carriers in the downlink, one possibility could be to have a one-to-one mapping between downlink and uplink component carriers such that data transmission on downlink component carrier n is acknowledged on uplink component carrier n. However, it cannot be assumed that the same number of component carriers is used in uplink and downlink. Rather, on the contrary, the most likely scenario is to have a larger number of downlink component carriers than uplink component carriers as the need for high data rates is expected to be greater in the downlink. Thus, transmitting multiple hybrid-ARQ acknowledgement messages, one per component carrier, can in some situations be troublesome.
Introducing a multi-bit hybrid-ARQ acknowledgement format is another possibility. However, transmission of multiple bits for hybrid-ARQ typically reduces the uplink coverage since energy per bit, or signal-to-noise ratio (SNR) target, decreases as more bits are transmitted. Furthermore, the capacity of the control signalling is degraded, both due to the increase in inter-cell interference and due to the increased amount of (time-frequency) resources needed in a cell to transmit multiple bits.
The current LTE specification has the possibility to transmit acknowledgement messages of up to two bits. This is used to support spatial multiplexing (MIMO) in which case two transport blocks on a single component carrier need to be acknowledged. In principle, this structure could be used for two separate component carriers instead. However, this solution is limited to at most two component carriers and, furthermore, does not allow the use of spatial multiplexing when multiple component carriers are scheduled.
Thus, it is necessary to find a solution to the problem of providing acknowledgements for each of the component carriers from the receiver to the transmitter without resorting to new control signalling formats or requiring multiple component carriers also in the reverse direction. Furthermore, there may be a need to improve the uplink control signalling coverage and capacity.